Equilibrium Fractionation of Oxygen Isotopes in the U3O8–Atmospheric Oxygen System

As a major component in the nuclear fuel cycle, octoxide uranium is subjected to intensive nuclear forensics research. Scientific efforts have been mainly dedicated to determine signatures, allowing for clear and distinct attribution. The oxygen isotopic composition of octoxide uranium, acquired during the fabrication process of the nuclear fuel, might serve as a signature. Hence, understanding the factors governing the final oxygen isotopic composition and the chemical systems in which U3O8 was produced may develop a new fingerprint concerning the history of the material and/or the process to which it was subjected. This research determines the fractionation of oxygen isotopes at different temperatures relevant to the nuclear fuel cycle in the system of U3O8 and atmospheric O2. We avoid the retrograde isotope effect at the cooling stage at the end of the fabrication process of U3O8. The system attains the isotope equilibrium at temperatures higher than 300 °C. The average δ18O values of U3O8 in equilibrium with atmospheric oxygen have been found to span over a wide range, from −9.90‰ at 300 °C up to 18.40‰ at 800 °C. The temperature dependency of the equilibrium fractionation (1000 ln αU3O8-atm. O2) exhibits two distinct regions, around −33‰ between 300 °C and −500 °C and −5‰ between 700 °C and −800 °C. The sharp change coincides with the transition from a pseudo-hexagonal structure to a hexagonal structure. A depletion trend in δ18O is associated with the orthorhombic structure and may result from the uranium mass effect, which might also play a role in the depletion of 5‰ versus atmospheric oxygen at high temperatures.


INTRODUCTION
The fractionation of oxygen isotopes ( 16 O, 17 O, and 18 O) among phases is an important measure applied in many scientific fields, such as geochemistry, 1 climate reconstruction, 2,3 and high-temperature magmatic and metamorphic processes, 4−6 to trace chemical reactions and environmental conditions. In the last 2 decades, several studies have investigated the use of oxygen isotopic composition in various uranium oxides as a signature for nuclear forensics applications. 7−17 Triuranium octoxide (U 3 O 8 ), the most stable form of uranium oxide, is a key compound in the uranium nuclear fuel cycle. U 3 O 8 is an intermediate product in the front end of the production cycle of the uranium nuclear fuel and a major compound in nuclear waste due to its thermal and chemical stabilities. 18−21 Understanding the mechanism governing the final oxygen isotopic composition of U 3 O 8 is fundamental to deciphering the material history in view of nuclear forensics investigations.
The production processes of U 3 O 8 in the nuclear fuel cycle differ among the various producers in starting materials and fabrication temperatures, mainly in the range of 500−800°C. 21 However, U 3 O 8 is manufactured under an atmospheric environment with full exposure to atmospheric oxygen.
A series of studies investigated the main parameters of the manufacturing process, which determine the final oxygen isotopic composition (δ 18 O) of U 3 O 8 . The effects of calcination temperature, calcination time, different starting materials, different initial solutions, air humidity, and the cooling rate of the products were studied. 7,11,12,15−17, 22 Plaue 11 synthesized U 3 O 8 from the U metal and different UO 2 samples and showed increased δ 18 O values with increasing temperature. Klosterman et al. 16 synthesized U 3 O 8 from uranium peroxide at 300−1000°C and reported a fractionation of about −22‰ up to about −5‰, respectively, between the oxide and atmospheric oxygen with a retrograde isotope effect. Klosterman et al. 22  were found to be similar and did not change as a function of calcination time or calcination temperature. Assulin et al. 17 showed that the cooling profile at the end of the fabrication process of U 3 O 8 determines the final oxygen isotopic composition, yielding a significant isotope effect in the order of 30‰, and that the interaction with atmospheric oxygen is the primary process that controls the δ 18 O value of U 3 O 8 . The common conclusion of the above studies is that the reaction with atmospheric oxygen is a crucial factor in determining the final oxygen isotopic composition of U 3 O 8 .
Exchange experiments provide a means to directly determine the fractionation factors or equilibrium constants. Early studies of kinetic and equilibrium isotope-exchange reactions between solid and gas 23 classified the exchange between U 3 O 8 and UO 2 and oxygen gas as the heterophase exchange but did not specify the mechanism through which the exchange occurs.
The isotopic exchange reaction involves changes in the vibrational frequencies of the lattice. The crystal structure influences the oxygen isotopic properties depending on the differences between the U−O bond energy and the interstitial sites occupied by oxygen atoms. 24 U 3 O 8 has different types of oxygen bonded to uranium, yielding different bond lengths. At room temperature (orthorhombic structure), two uranium atoms [labeled U(1) and U(2)] are surrounded by six oxygen atoms at distances between 2.07 and 2.23 Å, with the seventh oxygen atom bonded to U(1) and U(2) at distances of 2.44 and 2.71 Å, respectively. 25−28 Loopstra 29 established that the room-temperature orthorhombic and high-temperature hexagonal structures are similar, except that the hexagonal structure has one uranium site, whereas the orthorhombic structure has two. However, the surrounding oxygens are approximately equivalent for both lattice types. 29 The crystallographic structure of U 3 O 8 changes during the heating and cooling processes. There are three phases of U 3 O 8 having five related polymorphs. 30−34 There is a general agreement that U 3 O 8 transforms from an orthorhombic to a hexagonal structure depending on the temperature. Early work (Ackermann et al. 35 ) presented detailed data on the U 3 O 8 lattice parameters and concluded that the stoichiometry of U 3 O 8 changes continuously, reversibly, and anisotropically above room temperature, with phase transition from an orthorhombic to a hexagonal symmetry at 350 ± 10°C. On the other hand, later publications (Naito et al. 36 and Utlak and McMurray 25 ) determined that U 3 O 8 transfers from an orthorhombic to a hexagonal structure in two steps, a firstorder transition state from an orthorhombic to a pseudohexagonal structure at 295°C and a second-order transition state at 577°C to a hexagonal structure. A pseudo-hexagonal structure of U 3 O 8 at 500°C was also identified by X-ray diffraction and neutron powder diffraction (Herak 37 ).
Our study investigates the oxygen fractionation factor and the extent of the exchange reaction between U 3 O 8 and atmospheric oxygen in the range of 100−800°C. The cooling of the samples is performed under vacuum to prevent the retrograde process from altering the oxygen isotopic composition at each temperature. The measured crystallo-graphic changes at this temperature range are discussed in the context of the oxygen fractionation factor and the exchange reactions with atmospheric oxygen, which are involved in the fabrication process of U 3 O 8 .

MATERIALS AND METHODS
Three U 3 O 8 samples with different oxygen isotopic compositions were investigated in this study. Two were synthesized from uranyl nitrate hydrate (UNH); U 3 O 8 -I has a δ 18 O value of 10.02‰ ± 0.31‰, and U 3 O 8 -II has a δ 18 O value of 0.28‰ ± 0.13‰. These two materials were synthesized intentionally differently (at the cooling step) to achieve a difference in their δ 18 O values. 17 The third sample, U 3 O 8 -III, has a δ 18 O value of 4.78‰ ± 0.50‰, is a natural commercial U 3 O 8 material purchased from CETAMA, France (commercially known as "CHANTERELLE"), and is used as a calibration standard for impurities.
The samples (100 mg each) were calcined in a tungsten crucible and placed for 4 h in a quartz tube within a tube furnace at a temperature range of 100−800°C under an atmospheric environment. After 4 h at the desired temperature, vacuum was applied to the quartz tube (∼10 −3 Torr), and the oven was cooled to room temperature to avoid retrograde exchange reactions with atmospheric oxygen, which can shift the original isotope value. 16,17 XRD analysis (Rigaku, Ultima III) was performed on samples weighing several milligrams under an atmospheric environment by continuous scanning at 40 kV/40 mA in the range of 10−80°at a rate of 2°/min. The analyzed samples were those prepared at 100, 300, 400, 500, and 700°C. In addition, in situ XRD measurements at high temperature were performed on U 3 O 8 -II on a Smart-Lab (Rigaku, Japan) diffractometer equipped with a rotating Cu anode operating at 45 kV and 200 mA with a HyPix-3000 two-dimensional detector in Bragg−Brentano geometry with a variable incident slit. The beam, with a width of 5 mm, was shaped by 2.5°solar before and after the sample measurement. The detector was operated in the 1D mode with scattering and receiving slits equal to 5 mm. The powder was heated in situ with a DHS 1100 heating stage by Anton Paar in an ambient environment. The heating and cooling rates were 5°/min. The powder was kept at each temperature for 15 min before being measured. Diffraction patterns were measured between 2θ equal to 10− 100°with 1.5°/min. The DSC (differential scanning calorimeter�DSC823e, Mettler-Toledo) analysis was conducted on U 3 O 8 -III by weighing 4.68 mg into a 100 μL aluminum crucible. The measurement was performed under an atmospheric environment in the temperature range of 30−600°C at a heating rate of 10°C/min.
Oxygen isotopic analyses of the U 3 O 8 samples were conducted using an isotope ratio gas-chromatography-mass spectrometer (irmGCMS, Thermo Scientific Delta Plus Advantage) and an IR CO 2 laser (10.6 μm, New Wave Research 25 W). The method is described in detail elsewhere. 15,17 Briefly, U 3 O 8 samples (1005−1596 μg) and SiO 2 samples (NBS-28, standard material for quality check and calibration; 260−450 μg) were placed in nickel cups in a stainless steel chamber and heated overnight at 80°C under high vacuum. Pre-fluorination was performed thrice for the entire cell with 80 Torr of BrF 5 . Samples were reacted by laser heating in 90 Torr of a BrF 5 atmosphere. The liberated oxygen was purified by liquid nitrogen traps, concentrated on a 5 Å molecular sieve, cooled in liquid nitrogen, and transferred to the mass spectrometer through a gas chromatograph column for isotope measurement in a continuous flow mode. The international SiO 2 standard NBS-28 (δ 18 O = 9.58‰) 38 was used for consistency and calibration in each batch. The measured values are expressed in δ-notation in permil, relative to Vienna Standard Mean Ocean Water (VSMOW). The longterm standard deviation (SD) for NBS-28 was 0.30‰. Each U 3 O 8 sample was run at least in triplicate, and the SD is reported for each sample.  (Table 2 and Figure 3) show an orthorhombic structure in the temperature range of 25−200°C. At 300°C, two phases, orthorhombic and hexagonal, were obtained. At 400−800°C, a single hexagonal phase was obtained. The sample was cooled back to room temperature and then remeasured. The cooling back process yielded a single orthorhombic phase. These results present a reversible and continuous structural change, with expansion along the a-axis and contraction along the b and c axes, from the orthorhombic phase to the hexagonal phase at 300°C. These results agree with the results published by Ackerman et al. 35 and Miskowiec. 39 The crystallite sizes of the starting materials were calculated from the XRD measurements ( Figure 4). The crystallite size of the synthesized samples, U 3 O 8 -I and U 3 O 8 -II, remains stable and does not change as a function of the preparation temperature. In contrast, the crystallite size of the commercial material, U 3 O 8 -III, increases with the increase of temperature and decreases at 750°C. The average crystallite sizes of U3O8-I and U3O8-II are 80 and 70 nm, respectively, while U3O8-III has a larger average crystallite size of 132 nm.
DSC analysis was performed on U 3 O 8 -III. The results ( Figure 5) exhibit two exothermal peaks at 289 and 510°C associated with heat capacities of 10.16 J g −1 (8.56 J mol −1 , the blue area) and 163.08 J g −1 (137.13 J mol −1 , the red area), respectively. The second peak, at 510°C, indicates secondorder phase transitions. These results are in good agreement with the published literature. 36 However, Hideaki et al. 36 reported a heat capacity of 148 J mol −1 at 295°C and 314 J mol −1 , which might result from the heat capacity peak dependence on the heating rate.

DISCUSSION
The key observations following the experiments which were conducted are as follows: (1) the non-monotonic change in  (3) the occurrence of phase transition during the heating process. Thus, it is essential to evaluate whether the isotope equilibrium between U 3 O 8 and atmospheric oxygen was attained and, as a consequence, whether a fractionation factor at each temperature can be calculated.
Fractionation Factor and Percent of Exchange. In order to determine the fractionation factor between U 3 O 8 and atmospheric oxygen as well as its dependency on the temperature, we adopted the method developed by Northrop and Clayton. 40 It is based on the oxygen isotope exchange between two substances, considering the forward and reverse reaction rates and the isotope mass balance considerations. Equation 1 describes the partial exchange between a solid and a gas phase and allows the determination of whether the isotope equilibrium was attained or its proximity to the isotope equilibrium fractionation for a set of companion exchange runs (minimum of three samples), differing only by their initial where α is the oxygen isotopic fractionation between U 3 O 8 and atmospheric O 2 and K is the equilibrium fractionation of the system. Hence, a plot of (ln ∝ final − ln ∝ initial ) against (ln ∝ initial ) will give a straight line of slope B and an intercept of ln K. If equilibrium is attained in all samples of a set, ln ∝ final = ln K and B = −1. For samples that are not yet at the isotope equilibrium, B will lie between −1 and infinity. In addition, the percent of exchange can be calculated from the slope by (−100/B).
A set of three studied companion samples are shown to approach equilibrium from opposite directions at each temperature, satisfying the conditions required to apply this approach. The calculations (Table 3) are based on the data provided in Table 1 and a δ 18 O value of an atmospheric oxygen of 23.5‰. 41 The values of B (Table 3) for all temperatures indicate that the isotope equilibrium in the U 3 O 8 −atmospheric oxygen system was attained for the samples exposed to temperatures of 300−800°C. The calculated B values are very close to −1, as expected from the theoretical isotope formulation. 40 The calculated percent of exchange is also around 100%, corresponding to the B values. This conclusion coincides   Figure 6) show two distinct groups: the first at the temperature range of 300−500°C with values between −34.68 and −32.70‰ and the second at the temperature range of 600−800°C with values between −6.81 and −4.08‰. A sharp transition between these groups occurs between 500 and 600°C.
Such unique trends in the δ 18 O values of the U 3 O 8 samples were observed only in a few minerals, 24 suggesting that the structural change in the lattice is the main cause of that behavior.
The sharp change in the equilibrium fractionation factor coincides with the phase transition from the orthorhombic to hexagonal structure, which occurs at 500−577°C. 25,36,37 The XRD measurements showed a transition state from the orthorhombic to hexagonal phase at 300−400°C, close to the values reported by Ackermann et al., 35 Girdhar and Westrum, 42 and Notz, Huntington, and Burkhardt. 43 Apparently, some XRD measurements fail to identify the secondorder transition state, which is related to the presence of a pseudo-hexagonal structure that evolves from the orthorhombic structure. 28 In contrast to XRD, DSC measurements identified the two transition states at 295 and 510°C, which are related to the two transitions from orthorhombic to pseudo-hexagonal to hexagonal. Thus, we attribute the first-order transition state, at 300−400°C, to the fact that the δ 18 O values reach a minimum and the second-order transition state, at 500−600°C, to the inverse trend of the oxygen isotopic composition. We consider the possible isotope exchange with humidity H 2 O at 400°C (Klosterman et al. 22 ) minor to the isotope effect of the observed phase transition.
The temperature dependency of the fractionation factor is not common and does not resemble the published curves of most minerals of geological interest. The increase of 1000 ln α with increasing temperature up to 400°C, despite the full exchange for the orthorhombic phase, is unusual. This might be explained by the uranium mass effect, which is the tendency of heavy atoms (uranium) to favor the light isotope in order to lower the total free energy of the system by reducing the vibrational frequencies of the bonds. 24,44,45 Above 400°C, the fractionation factor is minimized with increasing temperature,

CONCLUSIONS
The isotope fractionation between U 3 O 8 and atmospheric O 2 was quantified by removing the retrograde effect during the cooling stage at the end of the fabrication process of U 3 O 8 from uranyl nitrate hydrate or heating U 3 O 8 under atmospheric conditions. We find that the U 3 O 8 −atmospheric oxygen system attains the isotope equilibrium at 300°C and maintained the isotope equilibrium conditions up to 750−800°C , where U 3 O 8 starts to lose oxygen. The temperature  dependency of the fractionation factor exhibits two distinct regions which correspond to the structural changes that U 3 O 8 undergoes at the temperature range of 25−800°C. The minimum of δ 18 O values which were obtained at 400°C is associated with the pseudo-hexagonal structure, and the second step, in the temperature range of 500−800°C, is a clear trend toward heavier δ 18 O which can be related to the hexagonal structure. The significant shift in the oxygen isotopic fractionation (α), in the temperature range of 500−600°C, coincides with the second-order transition state from the pseudo-hexagonal to the hexagonal structure. The fact that the δ 18 O value of U 3 O 8 does not reach the isotope value of atmospheric oxygen at the very high temperatures might be attributed to the uranium mass effect.